A 62-year-old male was admitted to our hospital in May, 1985, with double vision, which had persisted for 1 month. The neurological examination revealed left trochlear nerve palsy. A cerebral angiogram showed an arteriovenous malformation at the anterior cranial fossa. The malformation was mainly fed by the left anterior ethmoidal artery which branched off from the orbital branch of the left middle meningeal artery. There was a marked enlargement of anastomosis between the orbital branch of the middle meningeal artery and the recurrent meningeal branch of the lacrimal artery. Bilateral distal branches of the internal maxillary arteries, bilateral anterior falx arteries and the left fronto-orbital artery were also involved in supplying the AVM. The dilated fronto-orbital vein was the main drainer which emptied into the pterygoid plexus via the uncal vein and the sphenoparietal vein. In June 1985, the nidus involving the dura at the region of the cribriform plate and olfactory bulb was totally removed through a left frontal craniotomy. Postoperatively, the isolated left trochlear nerve palsy improved completely within a few days. This is the first reported case of isolated trochlear nerve palsy caused by a mixed dural-pial arteriovenous malformation of the anterior cranial fossa. We concluded that the etiology of isolated trochlear nerve palsy consisted of nerve compression due to the dilated orbital branch of the middle meningeal artery within the superior orbital fissure.